ArrayList中的removeIf

àì夳堔傛蜴生んèń 2021-12-21 09:39 245阅读 0赞

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# ArrayList #

# ArrayList中的removeIf #

ArrayList 继承了 AbstractList，实现了 List、 RandomAccess、 Cloneable、java.io.Serializable接口；  
AbstractList继承了 AbstractCollection，实现了 List接口；  
AbstractCollection实现了Collection接口；  
Collection接口继承了Iterable接口；  
Collection为顶级集合接口，实现了Iterable接口，可以迭代。

Collction接口中在Jdk1.8之后新增了个removeIf，有啥用？  
removeIf源码@since 1.8：

default boolean removeIf(Predicate<? super E> filter) {
           Objects.requireNonNull(filter);
           boolean removed = false;
           final Iterator<E> each = iterator();
           while (each.hasNext()) {
               if (filter.test(each.next())) {
                   each.remove();
                   removed = true;
               }
           }
           return removed;
       }

ArrayList对其进行重写：

@Override
       public boolean removeIf(Predicate<? super E> filter) {
           //首先要验证规则不能为空
           Objects.requireNonNull(filter);
           // figure out which elements are to be removed
           // any exception thrown from the filter predicate at this stage
           // will leave the collection unmodified
           //计数器，记录revove掉的次数
           int removeCount = 0;
           //声明要remove掉的集合，声明为set，保证不重复remove
           final BitSet removeSet = new BitSet(size);
           final int expectedModCount = modCount;
           final int size = this.size;
           //循环遍历list
           for (int i=0; modCount == expectedModCount && i < size; i++) {
               @SuppressWarnings("unchecked")
               final E element = (E) elementData[i];
               //boolean test(T t);校验参数是否匹配，匹配则返回true，如果匹配，则在removeSet中存入索引，removeCount值加一
               if (filter.test(element)) {
                   removeSet.set(i);
                   removeCount++;
               }
           }
           //校验此时的集合是否在多线程IO下受到改变，若受到改变则抛出异常
           if (modCount != expectedModCount) {
               throw new ConcurrentModificationException();
           }
           //源码中这句，表明了，将不需要移除的空间左移
           // shift surviving elements left over the spaces left by removed elements
           //定义anyToRemove，如果removeCount > 0则为true
           final boolean anyToRemove = removeCount > 0;
           //如果anyToRemove为true则继续进行，否则意味着没有匹配的可移除元素，直接return anyToRemove = false;
           if (anyToRemove) {
           //若有元素符合移除条件，则声明新size，重新定义list大小(ArrayList初始默认容量为10)
               final int newSize = size - removeCount;
               //循环遍历元素赋值，这里用到了一个新的方法nextClearBit(fromIndex)
               //将需要移除的元素在重新存储的时候跳过
               for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
               
                   i = removeSet.nextClearBit(i);
                   elementData[j] = elementData[i];
               }
               //如果新的集合与原集合比较，将指定要移除的设置为null,GC回收内存
               for (int k=newSize; k < size; k++) {
                   elementData[k] = null;  // Let gc do its work
               }
               //更新全局变量的集合大小为移除后的新集合大小
               this.size = newSize;
               //再次判断是否受到多线程IO影响
               if (modCount != expectedModCount) {
                   throw new ConcurrentModificationException();
               }
               modCount++;
           }
    
           return anyToRemove;
       }
       //官方解释：返回设置为在指定的起始索引上或之后出现的第一个位的索引。
        public int nextClearBit(int fromIndex) {
           // Neither spec nor implementation handle bitsets of maximal length.
           // See 4816253.
           if (fromIndex < 0)
               throw new IndexOutOfBoundsException("fromIndex < 0: " + fromIndex);
    
           checkInvariants();
    
           int u = wordIndex(fromIndex);
           if (u >= wordsInUse)
               return fromIndex;
    
           long word = ~words[u] & (WORD_MASK << fromIndex);
    
           while (true) {
               if (word != 0)
                   return (u * BITS_PER_WORD) + Long.numberOfTrailingZeros(word);
               if (++u == wordsInUse)
                   return wordsInUse * BITS_PER_WORD;
               word = ~words[u];
           }
       }
        private static int wordIndex(int bitIndex) {
           return bitIndex >> ADDRESS_BITS_PER_WORD;
       }

//验证结果

public static void main(String[] args) {
          List list = new ArrayList();
          list.add("Name");
          list.add("aaa");
          list.add("bbb");
          list.add("ccc");
          list.add("ddd");
          System.out.println(list.toString());
          //removeIf 的使用方法
          list.removeIf(next ->next.toString().indexOf("a")>0);
          System.out.println("===============removeIf 1====================");
          System.out.println(list.toString());
          //或者这样写
          Predicate<String> predicate = (s) -> s.equals("ddd");
          list.removeIf(predicate);
          System.out.println("===============removeIf 2====================");
          System.out.println(list.toString());
          List list2 = new LinkedList();
          list2.addAll(list);
          System.out.println("===================addAll================");
          System.out.println(list2.toString());
          List list3 = new LinkedList();
          list3.addAll(list);
          list3.clear();
          System.out.println("===================clear================");
          System.out.println(list3.toString());
          list3.addAll(list);
          list3.addAll(list);
          list3.removeAll(list2);
          System.out.println("===================removeAll 1================");
          System.out.println(list3.toString());
          List list4 = new LinkedList();
          list4.addAll(list);
          list2.add("test");
          System.out.println("===================removeAll 2================");
          System.out.println(list4.toString());
      }
      
      输出：
      [Name, aaa, bbb, ccc]
      ===============removeIf====================
      [aaa, bbb, ccc]
      ===================addAll================
      [aaa, bbb, ccc]
      ===================clear================
      []
      ===================removeAll 1================
      []
      ===================removeAll 2================
      [aaa, bbb, ccc]

可以看到：  
1.removeIf方法是给定一个判断条件，符合条件即删除。  
2.clear和removeAll的区别是：clear是无条件清空，removeAll是移除两个集合的交集。

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